1. Field of the Invention
This invention relates generally to a telecommunication method and apparatus and, in particular, to method and apparatus for achieving sample timing selection and frequency offset correction in conjunction with transmitting digital information in a Time Division Multiple Access (TDMA) cellular radio telephone system.
2. Description of Prior Art
Mobile radio involves communication via transmission of radio frequency signal between a mobile unit in a vehicle and a base station, or between mobile units. Time division multiple access (TDMA) digital mobile radio systems are described in the aforementioned "Frame/Slot Synchronization for U.S. Digital Cellular TDMA Radio Telephone Systems" by R. L. Toy and S. Chennakeshu, and "A. Bit Synchronization and Timing Sensitivity Viterbi Equalizers for Narrowband TDMA Digital Mobile Radio Systems", A. Baier, G. Heinrich, and U. Wellens, IEEE Vehicular Technology Conference, (Philadelphia), pp. 377-384, 1988. The communicating units must determine the beginning and end of signals intended for them, known as frame/slot synchronization. The complexity and accuracy of the frame/slot synchronization depend upon the number of points at which the signal is sampled. More samples per transmitted symbol implies greater accuracy at the expense of a higher complexity.
In order to keep the complexity reasonable for a practical implementation, fewer samples per symbol are employed for establishing frame/slot synchronization. The reduced number of samples, however, results in reduced accuracy with which frame/slot synchronization can be established. The resulting inaccuracy degrades receiver performance. Thus a more accurate sample timing scheme is required.
The transmitted signal may be reflected from physical and natural obstructions causing echoes which are received with the original transmission and is commonly known as multipath propagation. Multipath propagation further reduces receiver performance.
An additional problem with these systems is that the frequency of a local oscillator in the mobile station is likely to differ from that of the base station, and vice versa. The resulting frequency offset introduces a phase shift which causes the transmitted symbols to appear to be phase rotated at the receiver, thereby causing an ambiguity in the detection process. This severely degrades the performance of the receiver. In order to counteract this degradation in performance, it is necessary to estimate this frequency offset and apply a suitable correction to the received symbols.
Currently there is a need for a TDMA digital radio system which accurately selects sample timing for synchronization and corrects frequency offset in the presence of multipath propagation.